The present invention relates to methods and apparatuses for identifying cells in a cellular communication system.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. A proposal for such a new flexible cellular system, called Third Generation Long Term Evolution (3G LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth. However, it is expected that 3G LTE will be used not only for high rate services, but also for low rate services like voice. Since 3G LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will likely be the service that carries speech.
The physical layer of a 3G LTE system includes a generic radio frame having a duration of 10 ms. FIG. 1 illustrates one such frame 100. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms.
One important aspect of LTE is the mobility function. Hence, synchronization symbols and cell search procedures are of major importance in order for the User Equipment (UE) to detect and synchronize with other cells. To facilitate cell search and synchronization procedures, defined signals include primary and secondary synchronization signals (P-SyS and S-SyS, respectively), which are transmitted on a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH), respectively. The P-SySs and S-SySs are each broadcast twice per frame: once in sub-frame 0, and again in sub-frame 5, as shown in FIG. 1.
The currently proposed cell search scheme for LTE is as follows:
1. Detect one out of three possible P-SyS symbols, thereby indicating the 5 ms timing and the cell ID within a currently unknown cell group.
2. Detect frame timing and cell group using the S-SyS. This in combination with the results from step 1 gives an indication of the full cell ID.
3. Use the reference symbols (also called CQI pilots) to detect the cell ID. The interested reader is referred to the document R1-062990, entitled “Outcome of cell search drafting session”, TSG-RAN WG1 #46bis, Oct. 9-13, 2006 for more information about this proposal.
4. Read the Broadcast Channel (BCH) to receive cell-specific system information.
The SyS signals transmitted on the S-SCH are constructed as a pair of sequences, S1, S2 (see FIG. 1). The sequences are defined in the frequency domain. The signals to be transmitted on the S-SCH should be constructed such that the SyS pair S1, S2 should uniquely define the cell group and 10 ms frame timing once detected by the UE such that the cell group pn-sequence is detected and the UE can start the verification step (stage 3) of the above-described process (i.e., verification of the cell ID detected from stage 1 and stage 2 processing).
Furthermore, in order to minimize interruption time when performing Inter-frequency and Inter-Radio Access Technology (InterRAT) measurements, it is desirable that it also be possible to detect the cell group using only one SyS (i.e., S1 or S2 alone).
Consequently, there is a need for an S-SyS sequence design that will satisfy both requirements.